Digital acoustic volume indicator

ABSTRACT

This disclosure describes a digital volume indicator that replaces a conventional VU meter for speech level measurement. The indicator operates on speech-produced peak voltages. A voltage function analogous to the dynamic meter movement characteristic of the conventional VU meters is quantized in a series of threshold detectors. Suitable gating provides for a detector output only for the highest threshold exceeded for a given analog voltage excursion. Over a specified time, the detector outputs representing the quantized peaks are characterized as by averaging in an arithmetic section to produce a representative VU reading for the speech analyzed.

[72] Inventor Verne E. Munson Primary Examinerl(athleen H. ClatfyBrielle, NJ. Assistant ExaminerHorst F. Braun'er [2|] Appl. No. 824,142Attorneys-R. J. Guenther and Edwin B. Cave [22] Filed May 13, 1969 [45]Patented July 20, 1971 [73] Assignee Bell Telephone Laboratories,Incorporated Murray Hill, Berkeley Heights, NJ.

[54] DIGITAL ACOUSTIC VOLUME INDICATOR APSTRACT: This disclosuredescribes a digital volume in- 9cmms7nmwing Figs I dicator that replacesa conventional VU meter for speech 1 level measurement. The indicatoroperates on speech- U-S. N, roduced eak voltages A voltage functignanalogous to the 179/1 VL dynamic meter movement characteristic of theconventional ll- Cl. meters-is quantized in a eries of thresholddetectors Mot l gating provides for a dgtector output only for the VL;181/ 5; 32H77 B, highest threshold exceeded for a given analog voltageexcursion. Over a specified time, the detector outputs representingReferences Cited the quantized peaks are characterized as by averagingin an UNITED STATES PATENTS arithmetic section to produce arepresentative VU reading for 3,483,941 12/1969 Brady 181/.5 the speechanalyzed.

0 E WE llw l2 l3 I4' 15w E l SE TW i READOUT STAGE ANALOG ccr DETECTORumr SPEECH r SIGNAL DIGITAL ACOUSTIC VOLUME INDICATOR FIELD OF THEINVENTION This invention relates generally to the field of speechcharacteristic measurement; and particularly concerns the measurement ofspeech level.

BACKGROUND OF THE INVENTION In telephony as well as elsewhere in audioengineering it frequently is necessary to measure the levels of speech,as, for example, speech levels occurring on telephone lines. Theinstrument currently used to this end is termed a volume indicating (VU)meter in which the meter movement is a D'Arsonval galvanometer. Readingsof the present VU meter involve visually observing a series of meterneedle peak deflections in response to a speech sample, and thenestimating the average needle peak deflection. These procedures,however, subject the results to potentially wide variations even asamong comparably trained operators. The requirement of an attendant isin itself a disadvantage. Moreover, as the output is not digital, aconversion to digital notation would be required for any further machineprocessing.

Accordingly, one principal object of the invention is to develop avolume indicator which automatical form.

Another object of the invention is to reduce or eliminate the relianceupon operators in the process of measuring speech level.

A more specific object of the invention is to facilitate the automaticreading of speech level measurements in a manner suitable for digitalquantizing.

In the design of a digital volume indicator, it is desirable toduplicate as nearly as possible the ballistic characteristics of theDArsonval galvanometer, including the damping and overshootcharacteristics.

Additionally, since operators of VU meters have been instructed torecognize only peak deflections of the needle in the earlier instrument,the readout of the digital volume indicator should for compatibilityssake be based upon a comparable process.

Accordingly, a specific object of the invention is the generation of anelectrical signal with some characteristic which is the substantialequivalent of the VU meter needle position.

A further specific object of the invention is the electrical detectionof speech signal peak values which correspond to successive peakdeflections of a meter needle.

An additional inventive object is the combining of a series of peakamplitude values into a single value representative of the averagespeech level.

SUMMARY OF THE INVENTION In broadest terms, the invention uniquelycombines a meter analog circuit, a level detector circuit and acomputation unit. In response to speech input, the analog circuitgenerates a parameter that has a characteristic that varies in time in amanner closely similar to a meter needle movement. A series of detectorsfollow increases in that parameters value as it exceeds successivethresholds. Only the highest exceeded threshold is selected as thatstages output. The latter is converted to a digital code correspondingto the relative value of the speech level of the exceeded threshold.

In the computation unit, the codes are each converted into serial pulsesnumerically equal to the assigned value of the corresponding thresholdlevel. The total number of pulses are stored for some selectedmeasurement period. Concurrently, the number of codes received are alsostored. Division by the latter total of the fonner, at the end of themeasurement period, provides a measure of the average threshold levelexceeded, and thus also the average speech level in the period.

The invention proceeds in part from a recognition that the torqueequation that approximately describes the meter movement of theDArsonval galvanometer is analogous in form to the familiar second orderdifferential equation for charge stored in a capacitor in a series RLCcircuit.

In a particular embodiment, one detector is provided for each volumeunit in the range 9 to +3, which corresponds to the range in mostexisting volume indicator meters. Also, it has been found that, for theapplication at hand, the most satisfactory formula for converting asuccession of output pulses to a single volume unit value involves astraight averaging of all peaks. Other means for deriving a compositemeasure from the voltage peaks of the desired volume unit value caninclude weighting, discounting, selection of the median or mean pulse,etc.

A fuller understanding of the invention and its further objects,features and advantages will be gained from a reading of the detaileddescription to follow of an illustrative embodiment.

THE DRAWING FIG. 1 is a functional block diagram of the successive stepsand circuits which comprise the inventive embodiment;

FIGS. 2-4 are circuit diagrams of meter analog circuits;

FIG. 5 is a functional block diagram of the level detector unit;

FIG. 5a is a graph depicting the voltage vs. threshold response for thelevel detectors; and

FIG. 6 is a functional block diagram of the arithmetic unit.

DETAILED DESCRIPTION OF AN ILLUSTRATIVE EMBODIMENT A general descriptionof the overall design of the digital volume indicator is afforded by theblock diagram of FIG. 1. The input stage 11 receives speech signalspresent, for example, on a telephone line, providing high inputimpedance and amplification, as well as full-wave rectification toconvert the AC current speech signal into pulsating DC The outputsignal,

of stage 11, denoted E is delivered to analog circuit 12 which generatesan electrical function, for example, a voltage analogous to volumeindicator meter needle movements.

The voltage function in analog circuit 12 is monitored and quantized bylevel detectors in unit 13. There typically are 13 level detectors inunit 113 which cover a l3db. range in ldb. steps. For each occurrence ofan analog voltage peak, unit 13 generates an output indicative of thepeak s magnitude. These are converted in arithmetic unit I4 into binaryform and stored. Also, each level detector output is recognized as anevent; and the number of such events are also stored. The magnitudes ofthe several level detector outputs are totaled and divided by the numberof events, which yields for the time span in question an average peakamplitude reading. The latter is then fed to the readout section 15which provides temporary storage and conversion from binary to decimalformat for the quotient counts. The latter, which is the measure of theinput signal in volume units, are displayed in numerical form insuitable VU increments.

D'Arsonval Meter Analog Circuit By way of background, the needlemovement of a DArson: val meter can be approximately described by thefollowing torque equation:

(#9 d0 S J -i-D +S9- I (1) where:

J z moment of inertia D air plus circuit damping coefficient S 5 springconstant 0 angular displacement K E instrument constant I E currentEquation (l) in form is analogous to the familiar second orderdifferential equation for charge stored in a capacitor in a series RLCcircuit, such as shown in FIG. 2. For this circuit the charge equationis q q E dt C where:

qEthe instantaneous charge in the capacitor. In equations l and (2), thestored charge q can be regarded as the analog of 0, the needles angulardisplacement.

For the circuit of FIG. 2, a solution for equation (2) is realized byconsidering the necessary constraints to determine values of the analogcircuit.

In particular, sudden application of a single frequency signal ofamplitude such as causes a zero final needle position shall result in aneedle deflection to 99 percent of the final value within 3001-30 msec.Secondly, overshoot shall be restricted to a minimum of 1.0 percent andmaximum of 1.5 percent.

The FIG. 2 circuit, however, requires an impractical inductor withconstant L and R over an unduly wide frequency range. The desiredfunction can nevertheless be realized through the use of RC circuitry incombination with a unity gain amplifier with extremely high inputimpedance. Such a circuit is depicted in FIG. 3 and may be analyzed asfollows:

The Laplace transformation for a unit step function applied to the RCintegrator network R,C is given:

where S is the complex frequency operator.

If a second integrator network consisting of resistor R and capacitor C,is cascaded to the first network via a unity gain buffer amplifier A,,and assuming values of R,=R =R and C,= C C, then the voltage output e,is given by:

(SRC-l-l) (SRC+1) (4) When the inverse Laplace transformation isapplied, this equation may be rewritten as:

Now, it will be r'ecb' fi i z h that the critical damped solution forequation (2) is:

R 1 where q, :charge in the steady state and 2 I; Z6 Equation (6) leadsto the solution of a by imposing the constraint that the VU meterdeflection (or equivalently, the capacitor charge) should reach 99percent of its steady state value after 300130 msec. of single frequencytone is applied.

It will be noted that equation (5) is identical in form to that of thecharge equation (6). If the RC product of equation (5) is made to equall/a, then the output voltage E of the FIG. 2 circuit and the outputvoltage e, of the circuit of FIG. 3 will be identical.

A closer matching to actual VU meter responses is achieved by thecircuit of FIG. 4. This circuit is constructed by feeding the output e,of the FIG. 3 circuit through the circuit consisting of unity gainamplifier A, and the RLC network consisting of resistor R and inductor Lin series relation and capacitor C, in parallel relation therewith.

Thus, in the analog unit an electrical function is generated that isanalogous, i.e., linearly related or identical in responsecharacteristic, to a derived volume unit meter deflection function. Theelectrical function advantageously is a voltage produced in accordancewith the preferred circuitry of FIG. 4. Laboratory measurement of theresponse of the analog circuit of FIG. 4 has been compared with acomposite response curve constructed by averaging plotted points forseveral conforming volume unit meters; and the correspondence betweenthe two curves has been found to be good. Level Detection The analogvoltage function from circuit 12 is fed to a parallel array of thresholdor level detectors depicted in FIG. 5. Only three detectors 20, 21, 22are shown. Advantageously, a total of thirteen threshold detectorsprovide a threshold for each volume unit in the range 9 to +3, whichrange corresponds to the scale on most VU meters.

As the threshold of a given detector such as 20 is exceeded in responseto an increasing voltage input, that detector generates an output. Forexample, as illustrated also in FIG. 5a, detector 21 responds to ananalog voltage level which corresponds to a VU level of X, the generalcase. Normally while the voltage input ascends toward the responsethreshold of detector 21, its output is zero. As the threshold ofdetector 21 is exceeded, its output shifts to a positive voltage denotedby the pulse 23. The width of pulse 23 is determined by the time thatthe amplitude of the voltage input exceeds the threshold of detector 21.The output of detector 21 is differentiated by. capacitor 25, whichresults in the positive pulse 27 at the time the threshold of detector21 is exceeded, and the negative pulse 28 when the input voltage fromcircuit 12 drops below the threshold of detector 21.

In similar fashion, each of the 13 level detectors, including thedetectors 20 and 22 which are shown, respond to a particular voltagelevel of the analog voltage function. The capacitors 24 and 26 performsimilarly to capacitor 25. By a network of steering diodes associatedwith each level detector, such as the diodes 29, 30, 31, 32, 33, 34positive pulses such as 27 are gated through line 35 to the controlflip-flop 36. The first positive pulse received sets flip-flop 36, whichthen applies a signal through line 16 to gate 17 and onto 37 to openeach of the output gates as for example, the output gates 38, 39 and 40associated with detectors 20, 21, and 22 respectively. As long aspositive pulses such as 27 are gated to flip-flop 36, however, thesteering diodes such as 30, 32 and 34 prohibit any inputs to the gates38, 39, 40.

When the analog voltage from circuit 12 starts to decrease, a negativeoutput such as pulse 28 is generated in the manner described. Thenegative pulse occurs first in that level detector which experienced thehighest exceeded threshold. If detector 21 were that detector, then thefirst pulse generated, namely pulse 28, passes through steering diode32, and into pulse-shaping inverter 42 which converts the negative-goingdifferentiated waveform into a positive-going square wave pulse 47suitable for subsequent logic operations. Inverters 41 and 43 performsimilarly. The shaped pulse 47 next feeds through the open output gate39 to output terminal 45.

The output pulse 47 is also gated through diode 49 and line 51 to resetflip-flop 36. Flip-flop 36 in response removes the signal from line 16to gate 17. The pulse 47 on line 51 is also applied to gate 17 tomaintain the signal status on line 37. At the end of pulse 47, gate 17applies a signal through line 37 to close the output gates. Since thegates are closed, no further output from the output gates occurs. Thusthe pulse 47 corresponding to the highest threshold exceeded is the onlyoutput pulse permitted until the analog voltage from circuit 12 beginstorise again.

The output of detector unit 13 is a succession of pulses on the 13output terminals, each pulse corresponding to'the occurrence of a peakin the analog voltage amplitude with the particular terminal designatingthe actual value of the peak analog voltage. For convenience in thesubsequent arithmetic operations, arbitrary values of 1 through 13 areassigned to the respective level detector output. Thus to the lowestlevel of the detector namely level detector 22 a numerical value of I isassigned; the next lowest level detector is assigned a value of 2 and soon, the highest level being assigned the value of 13. The severaloutputs of detector unit 13 are converted to binary form thus inconventional diode matrix 6.

With reference now to FIG. 6, the outputs of diode matrix 6 is a seriesof four-level codes, each code corresponding to one tion of gate 63 isin the present embodiment achieved through manual measure switch 66.

Additionally for each output generated by detector unit 13, a singlepulse developed in decoder 62 is fed through line 59 to binary orevent-up-counter 65, which stores a running total of the eventsreceived, equal to the number of occurrences of analog voltage peaks.

At the conclusion of the desired measuring period, as signified byrelease of measure switch 66, a conventional division of the contents ofaccumulator 64 by the contents of event-upcounter 65 is commenced.Division monitor circuit 67 enables transfer gate 68, allowingevent-up-counter 65 to transfer its contents to event-down-counter 69.Thereafter circuit 67 directs accumulator 64 and event-down-counter 69to countdown in unison. When counter 69 reaches zero, the all-zero gate70 pulses monitor 67, which in turn enables transfer gate 68 again andalso pulses counter 7]. Counter 71 stores the pulse.

Successive fillings of the event-down-counter 69 and countdown of thecontents of accumulator 64 against that of counter 69 result in pulsingof counter 71 for each such event. When the contents of accumulator 64is zero, all-zero gate 72 pulses division monitor circuit 67 to halt theprocess. The contents of counter 71, representing the quotient isutilizable as a recording in digital form for later machine processing.Alternatively, as in FIG. 6, the quotient is gated to numerical lampdecoder-driver 73 which drives visual display 74. It can thus be seenthat the contents of accumulator 64 have been divided by the contents ofevent-up-counter 65 by the process of subsequent subtraction and thatthe contents of counter 71 is the average of the peak values of theinput analog voltage. Of course, account must be taken in the decodingprocess, before display on the lamps, to reconcile the assignment of thearbitrary values 1 through 13 for the conventional meter scale range of9 to 1+3.

Persons skilled in the art will appreciate that numerous alternateschemes can be enlisted to achieve the required computations anddisplay. Further, the analog circuit l2 can assume other forms such asactive filters, while accomplishing the same ends. Operationalamplifiers can be adopted as level detectors in unit 13.

The spirit of the invention is embraced in the scope of the claims tofollow.

What I claim is:

1. Acoustic level indicating apparatus comprising:

first means for receiving acousto-electric signal peaks, de-

propriate level detector, third means for separately summing saidoutputs and the total peaks received, means for arithmetically dividingthe summed outputs by the summed total peaks received, and means forutilizing the resulting quotient.

2. Circuitry for measuring speech volume units comprising:

means for converting a received el egtr i cal speech signal into avoltage function linearly related to a selected volume unit meter needledeflection functio r a voltage peak etectorwis in g a plurality ofdiscrete level detectors with successively greater threshold responsesfor quantizing said voltage function and producing for each excursionthereof an indication of the highest said threshold excecded; means fortransforming each said indicatlon into a tram of pulses numericallyrelated to the value of the highest exceeded threshold; means forsumming all pulses in a selected succession of such trains, and forsumming the number of pulse trains selected;

means for dividing the pulse sum by said number of trains;

and

means for utilizing the resulting quotient.

3. Circuitry as in claim 2, wherein said converting means comprises acircuit consisting of first and second integrator networks cascaded viaa buffer amplifier of unity gain.

4. Circuitry as in claim 3, further including a loop consisting of asecond unity gain amplifier, a resistive element and an inductiveelement, said loop being connected to the output of said secondintegrator network, and a capacitive element connected in parallelrelation to said last-named output.

5. Circuitry as in claim 3, further comprising means for fullwaverectifying said received electrical signal prior to receipt thereof bysaid converting means.

6. Circuitry as in claim 5, wherein said level detector thresholds arecalibrated to correspond to whole number levels in a volume unit rangeof substantially 9 to +3 VU. Mal

7. Circuitry in accordance with claim 6, wherein said utilizing meanscomprises means for displaying the numerical value of said quotient.

8. Circuitry for measuring speech volume units comprising:

means for converting a rece iy ed electricalspeegh signal into a voltagefunction linearly relate to a selected volume unit meter needledeflection function;

a voltage peak detector comprising a plurality of discrete leveldetectors with successively greater threshold responses for quantizingsaid voltage function and producing for each excursion thereof anindication of the highest said threshold exceeded;

means for transforming each said indication into a train of pulsesnumerically related to the value of the highest exceeded threshold;

means for deriving, from the pulses and pulse trains so generated withina given time span, a composite measure reflective of the voltage peaksdetected; and

means for utilizing the composite measure.

9. Circuitry as in claim 8, wherein said utilizing means comprises meansfor numerically displaying the value of said composite measure.

1. Acoustic level indicating apparatus comprising: first means forreceiving acousto-electric signal peaks, detector means having pluraldiscrete levels corresponding numerically to uniform volume unit steps,second means for generating and storing a binary output unique for eachlevel in response to receipt of each peak by the appropriate leveldetector, third means for separately summing said outputs and the totalpeaks received, means for arithmetically dividing the summed outputs bythe summed total pEaks received, and means for utilizing the resultingquotient.
 2. Circuitry for measuring speech volume units comprising:means for converting a received electrical speech signal into a voltagefunction linearly related to a selected volume unit meter needledeflection function; a voltage peak detector comprising a plurality ofdiscrete level detectors with successively greater threshold responsesfor quantizing said voltage function and producing for each excursionthereof an indication of the highest said threshold exceeded; means fortransforming each said indication into a train of pulses numericallyrelated to the value of the highest exceeded threshold; means forsumming all pulses in a selected succession of such trains, and forsumming the number of pulse trains selected; means for dividing thepulse sum by said number of trains; and means for utilizing theresulting quotient.
 3. Circuitry as in claim 2, wherein said convertingmeans comprises a circuit consisting of first and second integratornetworks cascaded via a buffer amplifier of unity gain.
 4. Circuitry asin claim 3, further including a loop consisting of a second unity gainamplifier, a resistive element and an inductive element, said loop beingconnected to the output of said second integrator network, and acapacitive element connected in parallel relation to said last-namedoutput.
 5. Circuitry as in claim 3, further comprising means forfull-wave rectifying said received electrical signal prior to receiptthereof by said converting means.
 6. Circuitry as in claim 5, whereinsaid level detector thresholds are calibrated to correspond to wholenumber levels in a volume unit range of substantially -9 to +3 VU. 7.Circuitry in accordance with claim 6, wherein said utilizing meanscomprises means for displaying the numerical value of said quotient. 8.Circuitry for measuring speech volume units comprising: means forconverting a received electrical speech signal into a voltage functionlinearly related to a selected volume unit meter needle deflectionfunction; a voltage peak detector comprising a plurality of discretelevel detectors with successively greater threshold responses forquantizing said voltage function and producing for each excursionthereof an indication of the highest said threshold exceeded; means fortransforming each said indication into a train of pulses numericallyrelated to the value of the highest exceeded threshold; means forderiving, from the pulses and pulse trains so generated within a giventime span, a composite measure reflective of the voltage peaks detected;and means for utilizing the composite measure.
 9. Circuitry as in claim8, wherein said utilizing means comprises means for numericallydisplaying the value of said composite measure.